A polyvinyl chloride resin foam has the advantages that it excels in fire retardance, chemical resistance, weatherbility and waterproofness and can be applied at a relatively low temperature, but is handicapped by the drawback that it has low heat resistance. It is known that this difficulty can be resolved by developing a cross-linked polyvinyl chloride resin foam. To date, however, such cross-linked polyvinyl chloride resin foam is actually produced only by the so-called pressurized foaming process such as a high pressure foaming process using a pressure-resistant sealed metal mold or high pressure inert gas process, (set forth, for example, in the Japanese patent publication No. 22,370/64 and Japanese patent publication No. 12,632/66). These proposed processes are inevitably carried out by the batch system, failing to produce the intended foam continuously. To obtain a sheet of cross-linked polyvinyl chloride resin foam, therefore, it has been necessary to slice a batch-produced foam block into a thin sheet.
Consequently, the conventional method of manufacturing cross-linked polyvinyl chloride resin foam which is based on the pressurized foaming process has been accompanied with the drawbacks that said conventional method involves costly manufacturing equipment; is handicapped by low productivity; moreover has to take many steps including, for example, the slicing step; the product becomes unavoidably expensive; and its market still fails to be expanded due to competition by less expensive foamed products such as foamed polyurethane, foamed polystyrene and foamed polyethylene.
For a long time, therefore, studies have been conducted on the method of continuously manufacturing a cross-linked polyvinyl chloride resin foam. To date, however, no process has been proposed which proves fully successful from the practical point of view.
Though not clearly understood, the failure to develop the method of continuously manufacturing cross-linked polyvinyl chloride resin foam at a low cost is inferred from the experiments carried out by the present inventors which complete this invention to have resulted in determining the undermentioned facts. Polyvinyl chloride resin tends to be thermally decomposed by the heat during kneading or molding and give forth hydrogen chloride gas. To avoid this objectionable event, it is always necessary to blend a stabilizer with the polyvinyl chloride resin. Normally, the stabilizer is applied in an amount of about 2 to 5 PHR. Where it is attempted to mix a thermally decomposable blowing agent such as azodicarbonamide with polyvinyl chloride resin, plasticizer and stabilizer by a mixing roller or Banbury mixer, then said blowing agent vigorously decomposes itself even at a lower temperature than the normal decomposition temperature of said blowing agent and evolves gases, making it impossible to obtain a unfoamed polyvinyl chloride resin molding.
Namely, it is impossible to provide a polyvinyl chloride resin composition, whether cross-linked or not, in which a thermally decomposable blowing agent, stabilizer and plasticizer are uniformly blended and is once gelled, that is, a foamable polyvinyl chloride resin composition.
On the other hand, a soft polyvinyl chloride resin foam has been continuously produced by applying a thermally decomposable blowing agent as set forth, for example, in the Japanese patent publications Nos. 18,828/67 and 13,874/68. These proposed processes indeed make it possible to blend a thermally decomposable blowing agent with the polyvinyl chloride resin without giving rise to the premature decomposition of said blowing agent. However, these published processes are necessary to use extremely fine polyvinyl chloride resin powder or the so-called paste resin manufactured by the emulsion polymerization process. So long as the expensive paste resin is used, it is impossible to produce a cross-linked polyvinyl chloride resin foam which can compete with inexpensive foamed products such as polyurethane foam, polystyrene foam or polyethylene foam.
To attain the intended object, therefore, it is necessary to apply the ordinary inexpensive polyvinyl chloride resin which is produced in large quantities by the suspension polymerization process. Where, however, it is attempted to blend a stabilizer and thermally decomposable blowing agent with the ordinary polyvinyl chloride resin, then a serious difficulty arises that said thermally decomposable blowing agent undesirably decomposes itself during the blending process.
It is be understood that the above-mentioned fundamental problem has made it impossible to apply the ordinary inexpensive polyvinyl chloride resin in the manufacture of the conventional soft polyvinyl chloride resin foam, and made it indispensable to purposely use special expensive polyvinyl chloride resin powder, that is, paste resin. In this connection, it may be added that the paste resin normally remains in a state referred to as plastisol, is uniformly miscible with, for example, a blowing agent at a lower temperature than 100.degree. C. without being heated to a gelling temperature, and therefore can be uniformly applied to the surface of a substrate (for example, a piece of fabric) in the form of a sheet. In contrast, the ordinary polyvinyl chloride resin can not be rendered uniformly miscible with a blowing agent, unless heated to a higher temperature than the gelling temperature of said resin. It is supposed, therefore, that an inexpensive cross-linked polyvinyl chloride resin foam available for pratical application has not been developed for the reason given above.